1. Field of the Invention
The present invention relates to a measuring apparatus and method for radiosurgery/stereotactic radiotherapy alignment. More particularly, the present invention relates to a method and apparatus for precisely measuring the isocenter of a target volume for a linear accelerator ("Linac") based radiosurgery/stereotactic radiotherapy system, thereby also compensating for or correcting alignment errors during treatment planning, such as errors due to misalignment of room laser alignment systems and couch (or table) or gantry sag, shifts, or misalignments.
2. Description of Related Art
Historically, several Linac based techniques have been used for the delivery of high-energy photon beams as a means for radiosurgery/stereotactic radiotherapy. These various methods have been reviewed by E. B. Podgorsak, Physics for Radiosurgery with Linear Accelerators, in Neurosurgery Clinics of North America, Vol. 3, No. 1, January, 1992. As therein noted, the system of multiple co-planar arcs is by far the most prevalent. These techniques have also varied with regard to the method of stereotactic head fixation during treatment, wherein the aim is to accurately align a target volume (isocenter in the patient's head or body part) within the centers of rotation of the couch/gantry coordinate system of the linear accelerator. The most common methods employed include either fixation of the stereotactic head frame to the radiotherapy couch ("couchmount systems") or fixation to a rigid floorstand mounted in the revolving floor plate of the Linac radiotherapy couch ("floorstand systems"). See, e.g., Podgorsak, supra; U.S. Pat. No. 5,027,818.
In the practice of the art, it is generally accepted that accuracy requirements for delivery of a radiation beam to a target volume isocenter should have fractions of a millimeter of accuracy, and some authors of floorstand systems have reported such accuracy in mechanical localization. In addition, floorstand systems have generally relied upon the use of phantom test targets that are x-rayed as part of the simulation set-up with the Linac for localization. This method of calibration makes use of a phantom test target placed at a given stereotactic coordinate, and then placed at the center of rotation of the Linac couch/gantry system. The Linac is then used to take test target x-rays with the couch and gantry in several different positions of rotation for the purpose of accurately aligning the stereotactic reference system. Couchmount systems are generally not as accurate as floorstand systems and have relied upon fixed room laser lights projected on a phantom carriage device for target localization within the Linac couch/gantry coordinate system. See, e.g., U.S. Pat. Nos. 4,123,143 and 4,223,227.
Although head fixation to a floorstand mounted in the Linac floor turret has historically been regarded by many as the most reliable and accurate method, it greatly limits the accessibility to the patient's head. This is particularly noticeable for lesions in the posterior portions of the head that frequently require that the patient be treated in the prone position when using such floorstand-based systems. In addition, floorstand-based systems require that the gantry of the Linac be specially rigged to protect against the possibility of accidental collision of the gantry with the floorstand during the execution of any treatment plan. Couchmount systems, on the other hand, have the advantage of being able to treat a given patient with the potential of a full 360 degrees of gantry rotation and, thereby, allowing the patient to be treated in a natural supine position, while also avoiding the need for gantry collision protection. See, e.g., Podgorsak, supra; U.S. Pat. No. 5,107,839.
The couchmount systems are desirable for the above-noted reasons; yet they have a significant disadvantage in that patient head fixation and stabilization within the coordinate system of the couch/gantry rotations of the Linac are generally not as accurate as floorstand-based systems. This is due to sagging and/or tilting of the couch which can occur in couchmount systems when weight, as weight of a patient's body, is applied to the couch tabletop after initial alignment. Such positional shifts are a source of error in couchmount systems. Some have attempted to solve this problem by cumbersome methods of bracing the couch tabletop, or the use of fixed, intersecting laser beams arranged as intersecting lines and emitted from laser alignment devices attached to the ceilings and walls of the room in which the Linac is housed for the purpose of referencing the origin of the Linac couch/gantry axes of rotation. U.S. Pat. Nos. 4,223,227 and 4,123,143. Such laser lights are commonly employed in the art of radiation therapy, are also known in the art to frequently shift and require recalibration or alignment, and can introduce yet another source of error in target alignment within the couch/gantry coordinate system. Furthermore, such laser beams are usually 2-3 mm wide and can also reduce the accuracy of alignment because of the thickness of the alignment beams and parallax. This aforementioned method relies on the use of lasers for visual alignment and no method, until the Applicants' invention, has been developed for the use of lasers for precise measurement of distances as a means for target volume localization within the couch/gantry coordinate system of a Linac-based radiosurgery/stereotactic radiotherapy system.
Therefore, a need exists for an apparatus and method for precisely measuring the isocenter of a target volume for a Linac based radiosurgery or stereotactic radiotherapy system, particularly in couchmount systems.